1. Field of the Invention
The present invention relates to a developing device mounted on an image forming apparatus such as a copying machine and a printer, and particularly to a developing apparatus having a developer carrying screw.
2. Related Background Art
As a developing apparatus mounted on “an image forming apparatus using an electrophotographic technology”, developing apparatuses using two-component developers with very good electrostatic property are widely used conventionally. FIGS. 7A and 7B show a construction of an ordinary developing apparatus using a two-component developer, FIG. 7A is a sectional view, and FIG. 7B is an explanatory plane view.
Reference numeral 10 denotes a developer container, which houses a developer 11 constituted of a toner and a carrier. An opening is provided at a region of the developer container 10, which is in close vicinity to and opposes to a photosensitive member drum 1, and a developing sleeve 2, which is a developer carrier, is provided at the opening. The developing sleeve 2 is a hollow non-magnetic metal sleeve, which contains a magnet roller 3 being magnetic field generating means therein.
Reference numerals 5 and 6 denote developer carrying members, which are screws each with blade members being wound around a center shaft in a cylindrical shape at fixed pitches. A first screw 5 and a second screw 6 are placed substantially in parallel, and an inner wall 7 is provided between the first screw 5 and the second screw 6 to partition them so that the developer does not move between them.
Inner walls do not exist at both end portions in a longitudinal direction of the developer container 10 so that the developer can move between the first screw and the second screw. The first screw 5 and the second screw 6 carries the developer in opposite directions from each other, and therefore when the screws are rotated, the developer circulates inside without being interrupted as shown by the arrows in FIG. 7B.
The developer which is being carried by the first screw 5 is carried on a surface of the developing sleeve by a magnetic force of the magnet roller 3, and is carried toward the opening of the developer container 10 with the rotation of the developing sleeve 2. Reference numeral 4 denotes a developer regulating member, and by regulating the developer on the developing sleeve 2 to a proper amount, a uniform coat of the developer is formed on the developing sleeve 2. A magnetic brush of the developer carried by the developing sleeve 2 contacts the rotating photosensitive member drum 1 at a developing portion, and an electrostatic latent image is developed on the photosensitive member drum 1.
FIGS. 8A, 8B and 8C show the screw for carrying the developer, FIG. 8A is an outline view of the screw, FIG. 8B is a sectional view cut along a plane passing through a rotation centerline of the screw, and FIG. 8C is an explanatory view of a developer carrying state. Reference numeral 13 denotes a rotation centerline, reference numeral 14 denotes a rotary shaft, and reference numeral 15 denotes a blade wound thereabout in a spiral form. If an angle formed by the blade 15 with respect to the rotation centerline facing in the developer carrying direction is assumed to be θ, the angle θ is usually set at about 70 to 80 degrees in consideration of a draft angle or the like of a forming die because the carrying force-for the developer becomes larger when the angle θ becomes closer to the perpendicular.
However, such a problem as will be explained below sometimes occurs to the conventional construction.
As shown in FIG. 8C, following the rotation of the screw, the developer 11 receives a pushing force in a traveling direction by the blade 15, and therefore the developer 11 leans to a side of a surface of the blade 15 in the carrying direction. Then, the amount of the developer 11 decreases as it is away from the blade 15, and therefore it is carried in each of blade portions in the state as shown in FIG. 8C. Consequently, as viewed the longitudinal distribution of the developer 11 a portion with a large amount of the developer 11 and a portion with a small amount thereof are formed in accordance with the pitch of the blade 15.
Since the developer moves onto the surface of the developing sleeve 2 from the screw 5, and further moves to an opposing portion to the photosensitive member 1 following the rotation of the developing sleeve 2, unevenness of distribution of the developer by the aforementioned screw 5 causes unevenness of distribution of the developer on the surface of the developing sleeve 2, which results in unevenness of density of an image in a longitudinal direction of the photosensitive member 1 (this poor image will be called “unevenness of the screw pitch” hereinafter).
Such unevenness of the screw pitch easily occurs especially when the developing sleeve 2 and the developer carrying screw 5 are designed to be placed close to each other to make the apparatus compact.
Even if each of constructions such as placement relationship of the developing sleeve 2 and the screws, magnetic relationship of the magnet inside the developing sleeve, and a regulation state of the developer layer thickness by the regulating member 4 is optimized so that the unevenness of the screw pitch does not appear on the image, it is difficult to prevent the unevenness of the screw pitch completely, as long as the developer carried by the screw becomes uneven in distribution at the screw blade pitches.
Further, the developing apparatus with use of the two-component developer having the toner and the carrier varies in the toner density of the developer in accordance with the balance of the toner consumption and replenishment, and therefore the amount of the developer sometimes increases and decreases while the apparatus is operating. Especially, in the state in which the amount of the toner in the developing apparatus is small, it is a task of extreme difficulty to prevent the unevenness of the screw pitch completely.